At present, the market of optical disc apparatuses is growing, and there is a demand for a high-stability and high-quality reproduced signal in the optical disc apparatus. When information is extracted from an optical disc medium, the amplitude of a signal extracted by a pickup is altered if there is a defect in the optical disc medium, or dust, a fingerprint or the like is attached on the optical disc medium. The amplitude of a signal extracted by a pickup is also altered due to a stress on an optical disc apparatus (e.g., defocusing, tilt, etc.), or a variation in reflectance, modulation factor or the like of an optical disc medium itself.
Therefore, conventionally, a VGA (Variable Gain Amplifier) is provided before binary data is generated, so as to cause an amplitude value to be constant to stably generate binary data, thereby providing high-quality reproduced information. Also, it is necessary to detect an envelope of a signal extracted from a pickup so as to use the envelope for pickup focus position learning, medium record/unrecord determination, or the like. However, since the envelope of a signal extracted by a pickup cannot be reproduced from a signal whose amplitude has been caused to be constant after the VGA, conventional optical disc apparatuses detect an envelope from a signal before the VGA (see Patent Document 1).
It is contemplated that an ADC (Analog-to-Digital Converter) is added after the VGA so as to exploit a digital signal processing technique, such as PRML (Partial Response Maximum Likelihood) or the like, and thereby further improve the accuracy of binary data. Further, as a miniaturization process progresses, the circuit scale of an analog portion tends to be dominant as compared to the circuit scale of a digital portion. Therefore, a digital circuit configuration leads to a reduction in cost.
Patent Document 1: Japanese Patent Unexamined Publication No. 2001-243714